How can I highlight all fonts at once on a PDF [IMPOSSIBLE TO CHANGE FONT IN UNDER 1 hour]

I have a PDF will multiple paragraphs of text, some with headers. I want to highlight ALL of the text and change the font all at once. Currently I can only highlight line by line when I use the type tool. Some times it even only lets me highlight a few words at a time. I would take me HOURS to change all the font on one page!